1. Field of the Invention
The present invention relates to an integrated quartz oscillator circuit having a capacitive voltage divider, which has a first capacitor and a second capacitor, as well as having a first transconductance amplifier, which is placed in a loop connecting a first junction and a second junction of the first capacitor.
2. Description of the Background Art
Quartz oscillators are used in electronic systems to provide system-internal reference frequencies and/or clock signals. Oscillating crystals exhibit damping, which can be characterized in an electrical equivalent circuit as an ohmic resistor, which is in series with a series resonant circuit of a capacitor and an inductor. To produce a stable oscillation, the energy loss occurring during the oscillation of an electrical charge in the resistor must be replaced.
This occurs, for example, by the aforementioned quartz oscillator circuit, when the oscillating crystal, optionally still supplemented with a load capacitor, is connected parallel to the capacitive voltage divider of the circuit. This type of quartz oscillator circuit is known as a Colpitts circuit with both integrated and external load capacitors. The transconductance amplifier can be a simple transistor in a collector circuit, whose collector is connected to a positive supply potential, its emitter as the amplifier output to a center tap of the voltage divider, and its base as the amplifier input to a complementary terminal of the first capacitor. The capacitive voltage divider characteristic for the Colpitts circuit determines the fractional amount of a voltage positively fed back via the thus formed loop. In the Colpitts oscillator, a voltage across one of the capacitors is measured and a current dependent on the measured voltage is supplied to the other capacitor. This results in problems in a symmetrizing of the circuit, therefore in a symmetric circuit design, which reacts less sensitively to interfering signals, occurring in a ground connection. All approaches in which the oscillating crystal or a load capacitor connected to the oscillating crystal is connected to ground are sensitive to interfering signals at the ground node. It is therefore usually necessary in integrated circuits, which have still other circuit parts apart from an (asymmetric) quartz oscillator circuit, to provide a separate ground pin for the quartz oscillator circuit.